Cell surface markers and shed antigens are used in the diagnosis of several cancers. For example, the CA125 antigen, recognized by the OC125 antibody, is a tissue-specific, circulating antigen expressed in ovarian cancer. The CA125 antigen is encoded by the MUC16 gene, cloned by Lloyd and Yin. The full-length gene describes a complex tethered mucin protein present primarily in a variety of gynecologic tissues, especially neoplasms. OC125 and other related antibodies react with glycosylation-dependent antigens present exclusively in the cleaved portion of the molecule.
A serum assay can detect elevated levels of the circulating CA125 antigen in many epithelial ovarian cancer patients, and this antigen, derived using the ovarian cell line OVCA433, is recognized by the OC125 antibody (1-2). The detection of circulating CA125 in serum has proven to be a useful tool for the management of ovarian cancer patients and clinical trials (3-4). However, CA125 is neither sufficiently sensitive nor specific for general cancer screening (5-6). A variety of CA125 linked antibodies including VK8 and M11 have subsequently been defined as present on ovarian cancer cells (7-9). Although these antibodies have been used to develop serum assays and various other studies in ovarian cancer, they have significant shortcomings for clinical use in screening or tissue delivery. These antibodies are not useful as screening tools, nor can they detect the proximal residual MUC16 protein fragment after cleavage. This has limited their diagnostic and therapeutic applications.
For example, OC125, M11, and most other antibodies prepared against ovarian cancer cell extracts are directed at complex, glycosylation-dependent antigens. These antigens are exclusively present in the shed portion of MUC16 and cannot be employed to follow the biology of the proximal portion of MUC 16 and may not accurately reflect tissue distribution since the glycosylation patterns can vary substantially among tissues. Because the vast majority of MUC16-reactive antibodies, including OC125, react with the glycosylation-dependent antigen present exclusively in the cleaved portion of the molecule, the true distribution of MUC16 expression is not known (21). There is currently no antibody available to track the fate of the remaining MUC16 protein fragment after cleavage and CA125 release.
Thus, there remains a need for the identification of antibodies that are directed against sequences in the peptide backbone of MUC16, and that are useful for diagnosis and treatment of cancers in which MUC16 is expressed and/or overexpressed.